188 HUMAN PHYSIOLOGY. 



extensor muscles of the thigh when the ligamentum patellae is 

 struck between the patella and tibia. This reflex is best observed 

 when the legs are freely hanging over the edge of a table. The 

 patellar reflex is generally present in health, being absent in only 

 two per cent. ; it is greatly exaggerated in lateral sclerosis and in 

 descending degeneration of the cord ; it is absent in locomotor 

 ataxia and in atrophic lesions of the anterior gray cornua. 



2. Ankle-jerk or Ankle Reflex. If the extensor muscles of the leg 

 be placed upon the stretch and the tendo Achillis be sharply 

 struck, a quick extension of the foot will take place. 



3. Ankle-clonus. This consists of a series of rhythmic reflex con- 

 tractions of the gastrocnemius muscle, varying in frequency from 

 six to ten a second. To elicit this reflex, pressure is made upon 

 the sole of the foot so as suddenly and energetically to flex the 

 foot at the ankle, thus putting the tendo Achillis and the gastroc- 

 nemius muscle on the stretch. The rhythmic movements thus 

 produced continue so long as the tension, within limits, is main- 

 tained. Ankle-clonus is never present in health, but is very marked 

 in lateral sclerosis of the cord. 



The toe reflex, per one al reflex, and wrist reflex are also present 

 in sclerosis of the lateral columns and in the late rigidity of hemi- 

 plegia. 



Special Nerve Centers in Spinal Cord. Throughout the spinal 

 cord there are a number of spinal nerve centers, capable of being 

 excited reflexly and of producing complex coordinated movements. 

 Though for the most part independent in action, they are subject to 

 the controlling influences of the medulla and brain. 



1. Ciliospinal center, situated in the cord between the lower cervical 

 and the third dorsal vertebra. It is connected with the dilatation 

 of the pupil through fibers which emerge in this region and enter 

 the cervical sympathetic. Stimulation of the cord in this locality 

 causes dilatation of the pupil on the same side ; destruction of the 

 cord is followed by contraction of the pupil. 



2. Genitospinal center, situated in the lower part of the cord. This 

 is a complex center, and comprises a series of subordinate cen- 

 ters for the control of the muscular movements involved in the 

 acts of defecation, micturition, and ejaculation of semen, and of 

 the movements of the uterus during parturition, etc. 



